You'll need something to boost the voltage. 9VDC won't do much of anything. Remember the 450v rating on the cap is its maximum working voltage (the voltage it can handle before it fails) the actual voltage delivered is determined by the input voltage if you put 9V into a cap you get ~9V out, the design of the circuit and other components determine the voltage delivered, not the caps themselves..

You can use the flash circuit from a disposable camera, but beware!
It will charge up the cap to around 300 volts!
Also make sure to get the polarity correct, or the cap might explode on you.
The flash circuit usually runs on 1.5-3V, so powering it with 9v might kill it.
When you take the flash apart make sure to take the battery out without touching and circuits, then short the capacitor leads with something conductive to discharge the capacitor.